This invention relates to electrical contact materials which are used for electrical contacts employed in electrical apparatuses such as switches, breakers, contactors, and the like.
Electrical contact materials dealt with in this invention are particularly those made of Ag-Sn-Cd ternary alloys which are made by melting Ag, Sn and Cd, and which are internally oxidized. Those belong to a different category from those which are prepared by mixing Ag, SnO and CdO powders and powdermetallurgically sintering them.
Heretobefore, Ag-Sn oxides alloys in which Ag is a matrix and Sn, solute metal thereof is internal-oxidized to Sn oxides, are widely used as electrical contact materials for the electrical apparatuses of the kind mentioned above.
As a similar electrical contact material, Ag-Cd oxides alloys are also known, while electrical contact materials made of Ag-Sn oxides alloys are more extensively employed today in view of the prevention of pollution, since Cd is harmful to health.
However, since Cd oxides have an excellent refactoriness and are afforded with an excellent resistability against electric arcs produced at switching on and off of electrical apparatuses, and since a contact resistance of electrical contacts made of Ag alloys employing Cd oxides is stable as the oxides evaporate reasonably by heat produced with electric arcs, we can not totally deny the employment of Cd oxides.
In this view, the electrical contact materials provided by this invention are those ternary Ag alloys containing CdO besides SnO.
On the other hand, there is a problem in the manufacture of Ag-SnO-CdO alloy contact materials under this invention. That is, it is impossible to completely internal-oxidize a total amount of Sn by oxygen which penetrates from the outside of Ag matrix and diffuses into the inside of the matrix, if said Sn is more than about 5 weight % of the Ag matrix. This is a phenomenon commonly accepted by those skilled in this art. And, for example, it is described in the Information (registration No. 1-11) published by DODUCO of West Germany in April, 1966 that in Ag-Sn alloys containing more than 5% of Sn, this Sn can not be oxidized by an internal oxidation method. It is pointed out there that this is because of segregation layers of Sn oxides which are inevitably formed at outer surface areas of such alloys and retard oxygen to penetrate into the alloys for developing the internal oxidation in inner areas. As mentioned above, this has been conceived unanimously by those skilled in industries related to electrical contact materials.
In order to solve this problem, it becomes necessary for a successful internal-oxidation to employ auxiliary solute metals which have higher diffusion velocities or which are more capable to carry oxygen and to convey the oxygen more efficiently into deeper inner areas of Ag matrices. Such auxiliary solute metals are typically In and Bi.
There is issed U.S. Pat. No. 3,933,485 in which Ag-Sn-In system alloys are internal-oxidized for obtaining modern electrical contact materials, and in which In is used as an auxiliary solute metal for the successful internal-oxidation of the alloys. Said electrical contact materials which are more specifically consisted of 5-10 weight % of Sn, 1.0-6 weight % of In, and a balance of Ag, are internal-oxidized. They are one of the most excellent contact materials which are industrially used today.
Nevertheless, even when In or Bi which can perform well internal-oxidation assisting functions, as explained above, is employed as an auxiliary solute metal, it is not easy to internal-oxidize more than 5% of Sn evenly throughout its Ag matrix. It is sometimes observed that Sn oxides happen to segregate excessively at outer surface areas of the Ag matrix, and such segregation makes subscales which are air-tight, while a depletion layer of Sn oxides is consequently produced in inner areas of the Ag matrix.
It shall be noted also that since InO and BiO have a comparatively lower refractoriness, and are comparatively weak metal oxides, it has been desired long since to internal-oxidize Ag-Sn alloys without the employment of In or Bi, if possible.
It will be noted also that compared to ternary Ag-Sn-Cd alloys, Ag-Sn-In-Cd alloys and Ag-Sn-Bi-Cd alloys which are quarternary, are provided with lower electrical conductivities. In this respect too, it is preferable not to use In or Bi as auxiliary elements for the sake of internal-oxidation. Although Cd can readily be internal-oxidized in a Ag matrix, Cd exerts little influence over the internal oxidation of Sn when Sn exists in the Ag matrix at an amount more than 5%.